While conventional port fuel injection (PFI) gasoline engines have extremely low particulate emissions, newer gasoline direct injection (GDI) engines have higher particulate emissions comparable to diesel engines. Gasoline particulate filters (GPF) have been introduced to the automotive market for the exhaust systems of GDI engines to reduce particulate emissions. Unfortunately, fresh, newly installed GPF have relatively low initial filtration efficiencies.
The low filtration efficiency for a fresh or low-mileage GPF results from the need to establish cake filtration in the GPF as particulate matter removed from the exhaust stream builds up on the filter substrate. The transition from bed filtration to cake filtration is relied on to achieve high filtration performance with minimum back pressure and filter size. Once cake filtration is achieved, the filter will continue to work with very little change to filtration efficiency throughout the service life of the unit.
In automobiles, this transition must be achieved rapidly to meet certification testing requirements for particulate emissions. Due to the limited operating time available to automakers to condition cars ahead of certification testing (on the order of hundreds to thousands of kilometers, depending on the test), the accumulation of sufficient particles to establish cake filtration via normal vehicle operation is difficult to achieve during the certification testing window. Engineers currently design the filter to compensate for poor initial efficiency by sizing up the filter, increasing the porosity, modifying the pore size distribution, and other changes that improve initial filtration efficiency. These design-arounds create a host of new problems, including cost, packaging constraints, and challenges to effectively distributing the exhaust stream across the filter. Therefore, methods for rapidly achieving high filtration efficiency are needed to permit more efficient filter design and to meet certification testing limits.